The Radiological Physics Center (RPC) was established upon the recommendation of the Committee on Radiation Therapy Studies (CRTS), now the Committee on Radiation Oncology Studies (CROS), under sponsorship of the American Association of Physicists in Medicine (AAPM) to insure the correctness and consistency of radiation dosimetry among institutions involved in interinstitutional clinical trials. Review of dosimetry is accomplished by visits to institutions by a physicist, irradiation of various mail-out dosimeters, and by review of dosimetry on patients entered into clinical trials. When errors are discovered, the RPC cooperates with the institution to rectify the errors. To date there are 16 active clinical study groups and 4 intergroup efforts served by the RCP. The RPC is presently reviewing individual patient records for 50 protocols. Visits have been made to 233 institutions. Dosimetry discrepancies have been found at 90 institutions of which 74 have been resolved. Efforts are made to develop calculative methods and efficient techniques for measurements that are important to clinical trials. Experience gained in developing quality control procedures for one clinical trial is communicated to other clinical trial groups. This information and other knowledge gained are communicated to the radiological community.